Lung diseases as asthma and sarcoidosis are known or suspected to involve inappropriate local immune responses to ubiquitous inhaled antigens. To mprove therapies and develop early intervention strategies, the mechanisms down-regulating pulmonary immune responses must be defined. Physiologic pulmonary immune responses wane, even with repeated antigen exposure, but responsible mechanisms are undefined. Parenchymal pulmonary cells inhibit lymphocyte proliferation in vitro by multiple mechanisms; owever, whether lymphocytes are growth-inhibited transiently or permanently in the lungs in vivo is uncertain. To study pulmonary immunoregulation, we have developed an experimental model system in which antigen-primed C57BL/6 mice are intratracheally challenged with the T cell-dependent antigen sheep red blood cells (SRBC), inducing a response which is vigorous but, importantly, self-terminating. We have found that in these mice, lung lymphocytes appeared to be cell-cycle arrested; they proliferated meagerly spontaneously in vitro and in vivo and were refractory to IL-2 despite a high percentage of CD25 expression. Many lung lymphocytes underwent apoptosis. Lung lymphocyte numbers were increased by cyclosporine-A treatment in vivo, which is known to inhibit activation- induced cell death (AICD). Apoptosis was restricted to cells which were CD4-, CD8-, B220-, but most of which were CD3+. Another sizeable lung lymphocyte population was also CD3+, CD4-, CD8-, B220-, but did not show DNA fragmentation. The fate of this latter population, designated CD3+ double negative (CD3+ DN) cells, and its relationship to other lymphocyte populations, is unknown. Our findings, thus, demonstrate a novel method by which pulmonary immune responses can be limited. We hypothesize that activated T cells become at risk for Fas-mediated apoptosis due to cell- cycle arrest which is triggered by inadequate co-stimulation and by suppressive factors produced by pulmonary cells. These T cells down- regulate CD4 (becoming CD3= DN cells) and eventually other surface receptors, which might otherwise have delivered anti-apoptotic signals. By contrast, despite Fas expression, T cells which receive adequate co- stimulation & death-repressing signals avoid apoptosis due to sustained expression of anti-apoptotic Bcl-2 family members. This application will explore the relationship between cell-cycle arrest and apoptosis of lung lymphocytes using in vitro assays and in vivo analysis of immunocompetent, mutant, and transgenic mice. The Specific Aims are: (1) Determine whether pulmonary lymphocyte apoptosis is Fas-dependent. (2) Determine whether - pulmonary lymphocyte apoptosis is blocked by over-expression of anti- apoptosis genes of the Bcl-2 family (3). Determine whether pulmonary T cells are cell-cycle arrested (4) Determine which pulmonary APCs trigger or prevent lymphocyte apoptosis. (5) Determine which pulmonary lymphocyte subsets are committed to apoptosis. Our long-term goal is to use this in vivo model system to translate in vitro observations into a comprehensive understanding of physiologic immunoregulatory mechanisms. Understanding these mechanisms should lead to novel approaches to the control of immunologic lungs diseases and lung allograft rejection.